1. Field of the Invention
This invention relates, in general, to photocopying and, more specifically, to sheet transporting systems in copiers, printers, duplicators, and like devices.
2. Description of the Prior Art
Electrostatographic apparatus, such as copiers, printers, and duplicators, can have many different basic forms of construction. One type of apparatus known in the prior art has a transfer drum which was dimensioned to hold two receivers, such as copy sheets, at the same time. U.S. Pat. No. 4,712,906, which issued on Dec. 15, 1987 to the same assignee as the present invention, discloses such an apparatus. In the referenced patent, the transfer drum can handle one 11".times.17" copy sheet or two 81/2.times.11" copy sheets simultaneously. The ability to process two sheets at the same time increases the throughput speed of the machine when using the smaller sized sheets.
In the above device, however, when color copying and a two up transfer drum are employed, there is a time delay between when the first two copy sheets enter the transfer station and the time the first two copy sheets, with an image thereon, exit the transfer station in immediate succession, for transfer to the fusing station. The same time delay is repeated for each new set of copy sheets. This delay is the result of each copy sheet making more than one revolution, with the transfer drum, for transfer, in registration, all of the separate colors of the images to be copied onto the copy sheet. Usually one revolution is required for each separate toner to be applied to the copy sheet. An example of this, for three color toning, is that during the first rotation each sheet is toned with yellow, with magenta during the second revolution and with cyan during the third revolution, with said third revolution being the same revolution that the securing means, such as vacuum ports, release the copy sheet from the transfer drum. A disadvantage of such a process is that if the fusing station is operating at a slower throughput rate than the linear output rate of the transfer station, which is usually the case in a two up color transfer process, any attempt to slow down the copy sheets between the transfer station and the fusing station results in overlapping of sheets. This is especially true where the image fusing station is located only a short distance away, such as less than the width of two copy sheets, from the transfer station. A further disadvantage of such a process is that the individual copy sheets containing the finished color copy images are not delivered, for processing, uniformly to the major subsystems in the copier or paper. To compensate for this non-uniform rate, each subsystem must be constantly adjusted, in relation to changes in the other subsystems, in order to produce quality copy output. For example, if the speed of the fuser is increased, the amount of heat and release oil applied to the copy paper would be different than if the fuser were running at a slower rate of speed. To maintain consistency in the copies, a change in the temperature of the fuser would be required to compensate for such a change in speed. Thus, changing the speed of the fuser is not usually desirable without a corresponding change in the amount of heat and release oil applied to the copy sheet to compensate for such change in speed.
There are various methods that can be used to bridge the above problems. One solution is to provide a greater distance between the transfer station and the fuser station. Both sheets can then be placed upon an intermediate vacuum transport system, slowed down on said transport system and fed separately, at the slower speed, into the fuser station. This, however, increases the overall size of the copier. Another method presently used involves the use of a smaller drum that only retains one sheet of paper. This, however, prevents imaging 11".times.17" paper. Still another means would be to slow down the copiers transfer station so that it operates at the same slow speed as the fuser; however, this decreases productivity.